Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to connections between devices in computer systems. More particularly, the invention relates to connections between peripheral devices via Universal Serial Bus (xe2x80x9cUSBxe2x80x9d). Still more particularly, the invention relates to a dual mode differential transceiver for use in a USB to permit both full speed and high speed data transfers to occur.
2. Background of the Invention
Conventional computer systems usually require a variety of peripheral devices to be connected to the computer chassis by a cable. Examples of peripheral devices (often referred to simply as xe2x80x9cperipheralsxe2x80x9d) include the monitor, keyboard, external modem, plotter, printer, mouse, joystick, trackball, speakers and other devices. Traditionally, each of the peripheral devices connects to the chassis through any one of a variety of different types of cables. A chassis has a number of connectors typically on a rear panel. These connectors generally include serial connectors, parallel connectors, Small Computer System Interface (xe2x80x9cSCSIxe2x80x9d) connectors, and various other standard or custom connectors. Some devices connect via a serial cable to the serial connector, while other devices connect to the parallel connector. Other devices, like the keyboard, connect to special purpose connectors.
There are a number of disadvantages with this peripheral device connection scheme. For example, dedicated cables for each peripheral device can make connecting peripheral devices to the chassis cumbersome and inconvenient. Sometimes, the user will not have the particular cable necessary to connect a desired device.
Many peripheral devices require the installation of associated expansion cards into the computer chassis. This process is highly undesirable for many users who have no desire to open the cover of the chassis. Furthermore, once a conventional computer has been turned on and has completed its initialization process (xe2x80x9cboot-upxe2x80x9d), such computers normally will not communicate with any new devices that are connected post boot-up. That is, all peripherals must be connected to the computer before the computer boots-up.
To address these and other concerns, the Universal Serial Bus (xe2x80x9cUSBxe2x80x9d) standard was introduced. The USB generally provides a solution for attaching peripherals that address the above-noted problems while balancing performance and cost. Devices attached to the USB ports can include additional connections for attaching other USB devices. For example, a keyboard may connect to the chassis via a USB cable to a USB connection on the computer chassis. The keyboard also has one or more additional USB connections to permit a mouse to connect to the keyboard. Thus, USB-compatible peripheral devices can be linked together making connection of peripherals easier than with conventional computer systems. USB provides other benefits such as the ability to support xe2x80x9cplug and play.xe2x80x9d Plug and play means that a device can be connected to a computer that has already been booted-up and the device will automatically be configured and be ready for immediate use.
The current USB specification (USB 1.1) imposes a maximum data rate of 12 megabits per second (Mb/s). That is, the maximum rate at which two or more devices can communicate with each other over a USB cable is 12 Mb/s. While this maximum rate may be acceptable for a keyboard, in which the limiting factor usually is the substantially lower speed at which a human can type, the maximum rate can be a problem for higher bandwidth types of operations such as transmitting data from a scanner.
The connection path between a device transmitting data and another device receiving the data is a transmission line. The type of the transmission line is often characterized by the electrical termination. In accordance with well-known principles of electrical circuit theory, one or both ends of the transmission line usually include one or more passive electrical devices, typically resistors. The resistors xe2x80x9cterminatexe2x80x9d the transmission line to minimize signal reflections and improve signal integrity.
Data is transmitted across a USB cable in a digital format in which a voltage changes between two states, e.g., 0 volts and 3.3 volts. The change in voltage between these states is referred to as a signal transition. The USB standard requires xe2x80x9csourcexe2x80x9d termination which means termination resistors are connected in series with the output terminals of the transmitters (i.e., the source of the signal). Source termination, which is required by USB 1.1, results in a signal transition being transmitted through the USB cable and a portion of the energy that comprises that signal is reflected back to the sending device. Because of the reflection, the USB 1.1 serial termination scheme precludes the sending device from changing the state of signal to be transmitted over the cable until the reflection from the previous transition has reflected back to the sending device. Electrical signals propagate along a cable at a predetermined speed. USB 1.1 is specified for a maximum cable length of 5 meters. With this length of cable and given the amount of time a signal will take to travel down a 5 meter cable and back, the maximum data rate that is permissible under USB 1.1 is 12 Mb/s data. Data rates faster than 12 MB/s second will cause reflected signals to interfere with subsequent signal transitions and result in loss of data.
For some types of peripheral devices, it is highly desirable to be able to transfer data at rates faster than what is permitted by USB 1.1. At the same time, it would be desirable for the computer to still accommodate USB peripherals constructed in accordance with 12 Mb/s USB 1.1 protocol. This would permit both slower USB 1.1 devices or faster USB devices to be connected to the same computer system. Despite the advantages such a system would provide, to date no system is known which can accommodate USB 1.1 and devices capable of communicating at faster data rates.
The problems noted above are solved in large part by a computer system having a bus to which one or more devices, such as peripheral devices, connect. The bus may be, for example, a Universal Serial Bus (xe2x80x9cUSBxe2x80x9d) and the devices connected to it are USB-compatible devices. A USB host/controller controls the USB bus. One or more of the USB devices and/or the host controller have an electrical interface that includes two transmitters and a receiver for bidirectional data transmission. At least one transmitter preferably is a slower large amplitude, differential transmitter and the other is a faster differential transmitter in which the data signal is ground referenced. The faster transmitter can transmit data at a rate that is faster than the slower differential transmitter. The electrical interface also includes an electrical termination device that is disposed between the output terminals of the two transmitters. The termination device preferably comprises a pair of multi-purpose termination resistors that advantageously can provide serial termination or parallel termination depending whether the fast or slow transmitter is used.
When transmitting using the slower transmitter, which is used to communicate in accordance with the USB 1.1 specification, the transmitting USB device disables the output of its faster transmitter by deasserting an output enable (OE) signal to the faster transmitter. The receiving USB device disables all of its transmitters. In this slower communication mode, the termination device plus the output impedance of the slower transmitter provides serial termination and the data from the slower transmitter passes through the termination device to the destination device. When transmitting using the faster transmitter, both USB devices assert single ended zero (SE0) signals to their slower transmitters which forces both of their output signals to a low impedance state. In this latter slower transmission mode, the termination device, connected to the outputs of the slower transmitter which are forced low, provides parallel termination, effectively functioning as a xe2x80x9cpull-downxe2x80x9d terminator. With parallel termination at the destination end, echoes effectively are reduced or eliminated and faster data rates are thereby attainable. Further, a USB device with such a multi-purpose electrical interface is compatible with the existing USB 1.1 specification, but can also be used to transmit and receive at faster data rates than are permissible under the USB 1.1 specification.